There are several types of image sensors using charge coupled devices (CCDs): linear, frame transfer and interline image sensors. In frame transfer image sensors, each pixel is composed of a set of two or more adjacent polysilicon electrodes. Polysilicon is a partially transparent material. In such devices, an aluminum light shield patterned with openings corresponding to selected electrodes may be provided. Light shines through the opening passing through the polysilicon electrode into the image sensor.
Frame transfer type devices require a means of shuttering in order to reduce extraneous signals during the read out time to an acceptable level. Such extraneous signals are commonly referred to as "image smear". In order to avoid the need for mechanical shuttering and to reduce image smear interline transfer type CCD image sensors have been developed.
In interline transfer imaging devices, photogenerated charge is collected at photodiode sites (or pixels) on pn junctions or under the gates of photocapacitors, for a period of time and then transferred into adjacent charge coupled registers to be detected by an output circuit. In an area array of such photocharge collection sites it is necessary to transfer the collected photocharge, first to a vertical CCD shift register and then to a horizontal CCD shift register, finally, reaching a charge sensitive detector or amplifier.
Typically, a light shielding material is provided over the vertical and horizontal shift registers in order to prevent the still incident light from producing smear signals during the time for read out of the device. In this way, the need for a mechanical shutter is eliminated and the photocharge collection sites may continue to collect charges to be read out subsequently.
In prior art, such as disclosed in Oda, U.S. Pat. No. 4,772,565, for example, and indicated schematically in FIGS. 1 and 2, a given row of pixels 10 is addressed by application of a voltage to electrodes 20 and 30, composed respectively of first and second levels polycrystalline silicon (poly-1 and poly-2) and, which are both connected to the same vertical clock, .PHI..sub.1. Upon application of a voltage, photocharge is transferred to a buried channel 40 of the vertical shift register. Electrical isolation between photodiodes and the vertical shift register is provided by a channel stop region 15, also indicated in FIG. 1. As shown in FIG. 2, this vertical shift register is composed of buried channel 40, overlapping electrodes 20 and 30 which are connected to vertical clock .PHI..sub.1, and overlapping poly-1 and poly-2 electrodes 50 and 60 which are connected to vertical clock .PHI..sub.2. These electrodes are separated from the substrate semiconductor 70 by an insulating layer 80. The regions 65 beneath electrodes 30 and 60 are ion implanted to provide a potential energy difference between regions 25 and 26, controlled by the .PHI..sub.1 clock, and between regions 55 and 56, controlled by the .PHI..sub.2 clock.
Because the polysilicon electrodes are transparent to light, an additional layer of light shielding material must be provided to prevent extraneous photoinduced charges from interfering with the image charges during the charge read-out via the CCD shift registers. To provide such a light shield, an insulating layer is provided over the polysilicon electrodes and an opaque layer such as aluminum is deposited onto the insulating layer. This opaque layer is patterned with openings corresponding to the photodiodes but covers the polysilicon electrodes in the vertical CCD shift register. This added light shielding layer adds topographic variation to the device and provides additional opportunity for short circuits to the polysilicon gate electrodes, for example, through pinholes in the insulating layers beneath the aluminum layer.